The disclosure of Japanese Patent Application No. 2000-147760 filed on May 19, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a vehicle drive power control apparatus and a drive power control method.
2. Description of the Related Art
In vehicles equipped with an internal combustion engine and a continuously variable transmission, it is recently demanded to improve the fuel economy of the internal combustion engine while maintaining a requested value of the vehicle drive power.
To meet such a demand, a drive power control apparatus described in Japanese Patent Application Laid-Open No. HEI 11-198684, as for example, performs an overall control (coordinative control) of the torque of the internal combustion engine and the speed ratio of the continuously variable transmission (the input rotation speed thereof). That is, when adjusting the engine output so as to bring the vehicle drive power to a target value, the control apparatus controls the speed ratio of the continuously variable transmission so that the engine revolution speed (the input rotation speed of the continuously variable transmission) will become equal to a value that optimizes the fuel consumption, and at the same time, controls the torque of the internal combustion engine to a target value.
Furthermore, with regard to the internal combustion engine installed in the vehicle as described in the aforementioned laid-open patent application, the form of combustion can be changed between the lean burn and the stoichiometric burn. The combustion form of the internal combustion engine is controlled so as to achieve a combustion form that realizes a fuel consumption performance that is optimal to bring the vehicle drive power to the target value. That is, according to the aforementioned laid-open patent application, the fuel consumption performance in the lean burn operation and the fuel consumption performance in the stoichiometric burn operation are compared beforehand. When the vehicle drive power is to be brought to a target value, the control apparatus selects one of the combustion forms that will achieve a better fuel consumption performance, in accordance with the present state of engine operation.
In internal combustion engines that perform lean-burn operation, a NOx storage-reduction catalyst that adsorbs and stores oxides of nitrogen (NOx) is provided in the exhaust system. During the lean-burn operation, NOx in exhaust gas is adsorbed to the catalyst so as to lessen NOx emissions. Furthermore, during the lean-burn operation, a rich spike control of temporarily achieving a fuel-rich air-fuel ratio is performed to conduct a rich combustion, so that the NOx adsorbed by the NOx storage-reduction catalyst is reduced into nitrogen (N2) by hydrocarbons (HC) and the like present in exhaust gas. Thus, Nox saturation of the NOx storage-reduction catalyst is substantially prevented.
However, if the rich spike control.is performed during the lean-burn operation, the fuel consumption performance deteriorates by an amount of fuel consumption involved in the rich spike control, in comparison with the case where only the lean-burn operation is performed. Therefore, in the cases where the rich spike control is performed at a high frequency during the lean-burn operation, as for example, the fuel consumption involved in the rich spike control increases, so that in some cases, execution of the stoichiometric burn instead of the lean-burn operation would provide a better fuel consumption performance. If the fuel consumption performance during the lean burn and the fuel consumption performance during the stoichiometric burn are reversed in the superiority as mentioned above, the selection of a combustion mode based merely on the comparison between the fuel consumption performance achieved during the stoichiometric burn operation and the fuel consumption performance achieved during the lean-burn operation where the rich spike control is not considered involves a possibility of performing a combustion mode (lean burn) that is not favorable in fuel consumption performance and of failing to achieve a sufficient improvement in the fuel consumption performance of the internal combustion engine.
The invention has been accomplished in view of the aforementioned circumstances. It is an object of the invention to provide a drive power control apparatus and a drive power control method for a vehicle equipped with an internal combustion engine in which a rich spike control is performed during a lean-burn operation, the apparatus and the method being capable of precisely improving the fuel consumption performance of the vehicle.
Means for achieving the aforementioned object, and operation and advantages of the means will be described below.
To achieve the aforementioned object, a drive power control apparatus and a drive power control method in accordance with a first mode of the invention are applied to a vehicle that includes an internal combustion engine that changes a combustion form between a lean burn and a stoichiometric burn and that performs a rich spike control of temporarily shifting an air-fuel ratio to a fuel-rich air-fuel ratio during a lean-burn operation, and a continuously variable transmission connected to an output shaft of the engine. An engine output for bringing the drive power of the vehicle to a target value is determined as a target output, and the torque of the internal combustion engine is controlled so that the actual engine output reaches the target output. The control apparatus and the control method perform the lean burn when the target output is less than an output value set based on a minimum fuel consumption rate during the stoichiometric-burn operation and a minimum fuel consumption rate during the lean-burn operation in which a fuel consumption involved in the rich spike control is taken into account. The apparatus and the method perform the stoichiometric burn when the target output is greater than the output value.
According to the above-described construction, the output value that serves as a criterion for determining whether to switch the form of combustion is set based on the minimum fuel consumption rate during the stoichiometric-burn operation, and the minimum fuel consumption rate during the lean-burn operation in which the fuel consumption involved in the rich spike control is taken into account, instead of the minimum fuel consumption rate obtained when the lean-burn operation is simply performed. Therefore, when the actual engine output is controlled to the target output, a form of combustion that achieves a best fuel consumption performance can be precisely selected and performed by selecting a form of combustion in accordance with whether the target output is greater or less than the output value. Hence, in the vehicle equipped with the internal combustion engine in which the rich spike control is performed during the lean-burn operation, the fuel consumption performance of the engine can be precisely improved.
It is also practicable to determine the minimum fuel consumption rate during the lean-burn operation in which the fuel consumption involved in the rich spike control is taken into account, and the minimum fuel consumption rate during the stoichiometric-burn operation, while the vehicle is being driven, and to calculate the output value based on the two minimum fuel consumption rates.
According to this construction, the output value that serves as a criterion for determining whether to switch the combustion form is calculated during operation of the vehicle equipped with the internal combustion engine, so that the output value can be set to an appropriate value irrespective of individual variations of the vehicle, the internal combustion engine, etc. Therefore, by selecting and performing a combustion form that achieves a best fuel consumption performance based on the output value and the actual engine output, the fuel consumption performance can be improved more precisely.
The controller may perform the rich spike control so as to reduce a NOx stored in a NOx storage-reduction catalyst, and the controller changes an execution frequency of the rich spike control in accordance with a change in a NOx storing capability of the NOx storage-reduction catalyst provided in an exhaust system of the internal combustion engine. When changing the execution frequency of the rich spike control in accordance with a change in the NOx storing capability of the NOx storage-reduction catalyst, the controller may vary the output value in accordance with the change in the NOx storing capability of the NOx storage-reduction catalyst.
If the execution frequency of the rich spike control is changed in accordance with the NOx storing capability of the NOx storage-reduction catalyst, the minimum fuel consumption rate during the lean-burn operation in which the fuel consumption involved in the rich spike control is taken into account also changes. According to the above-described construction, the output value that serves as a criterion for determining whether to switch the form of combustion in accordance with the NOx storing capability of the NOx storage-reduction catalyst is variable, so that the output value can be set to an appropriate value even though the execution frequency of the rich spike control is changed. By selecting and performing the form of combustion based on the actual engine output and the variable output value, the fuel consumption performance can be improved more precisely.
The output value may be varied in accordance with the speed of the vehicle as well as the NOx storing capability.
In the control of the actual engine output to the target output P, the drivability during the lean-burn operation and the drivability during the stoichiometric-burn operation differ from each other. During the lean-burn operation, the drivability in a high engine output region deteriorates more as the vehicle speed V increases. According to the above-described construction, however, the output value that serves as a criterion for determining whether to switch the form of combustion is variable in accordance with the vehicle speed, which is a parameter that affects the drivability. Therefore, the form of combustion can be selected so that the drivability during the lean-burn operation will not excessively deteriorate.